In clinical analyzers for assaying patient sample liquids, it is conventional to provide a stack of test elements each of which is used to receive a portion of a patient sample liquid to do the assay, from a depositor. Such a stack features a plunger and a driver for the plunger, which press against one end of the stack to move its opposite end to a location for the dispensing of the test element into the analyzer. Means are also associated with the plunger, such as a bar code, for counting how far the plunger has moved, and hence how many test elements have been dispensed.
An example of this is taught by the slide cartridges and plungers therefor used in "Ektachem" analyzers, e.g., the "E250" analyzer. In such a case, a driver for the plunger comprises a complicated series of linking arms that pivot to convert rotary motion from a DC motor into what is almost linear motion of the plunger. A spring appropriately connected at one of the pivot points ensures an almost constant biasing force on the plunger.
Although such a mechanism has worked satisfactorily, it is less than ideal for several reasons. One is that its very complexity and large number of moving parts produces a low reliability. Another is all those parts lead to extra manufacturing costs. Still further, tolerance errors in all those parts can add up and thus lead to a departure from the desired "constant force" delivered by the plunger, or its desired linear movement. Finally, the linkage of the drive mechanism can produce vibrations which, because counting is via a bar code, produces a false count.
Thus, there has been a need prior to this invention to provide a simplified, truly linear drive to a plunger that contacts and pushes a stack of test elements, to more positively control the force exerted against the stack as well as the "count" that the mechanism measures of the remaining elements in the stack.